Blending two or more plastic polymeric materials to achieve certain specific properties has become an important technology. Such polymer blends can be thought of as macromolecular plastic alloys. The polymeric materials of the blend are chosen so as to optimize the properties of the blend. For example, a high glass transition temperature (T.sub.g) polymer characterized by exceptional toughness and poor solvent resistance might be blended with a polymer which has excellent solvent resistance but a lower T.sub.g, where the latter polymer alone exhibits poor mechanical properties, e.g., tensile and flexural modulus. The resulting blend would be expected to have properties intermediate those of the individual blend constituents, the particular properties dependent on the proportions of the constituents.
Polyesters having aromatic moieties, e.g., polyarylates, are often employed as at least one of the polymers in such blends. Polyarylates are high temperature, high performance polymers which have a good combination of thermal and mechanical properties. Additionally, polyarylates have inherent combustion resistance as well as good weatherability. They also have good melt stability at high temperatures and good color retention. Still further, they have good processability which allows them to be molded into a variety of articles. Polyarylates have been blended with a number of other polymers, such as ABS resins (U.S. Pat. No. 3,792,118), polycarbonate resins (U.S. Pat. No. 3,792,115), polyurethane resins, methacrylate resins, etc. When there is an extreme of viscosity difference between a high viscosity polyarylate and the other polymer, severe surface irregularities (e.g., jetting) are observed when these blends are injection molded. This is observed especially when polyarylate is blended with, for example, ABS resins and poly(methyl methacrylate) resins. Additionally, the extremely high viscosity of polyarylates prevents a uniform of product from being obtained by conventional polymer techniques, i.e., extrusion or Banbury type melt mixing, when the polyarylate is blended with a resin having a lower viscosity. The non-uniform blend will not weather as well as the uniform blend of the resins nor will it have an acceptable balance of properties. Still further, the inclusion of lower molecular weight constituents (which generally have lower melt viscosities) in order to lower the melt viscosity of the blend and make the blend more processable is generally at the expense of physical properties of the blend such as toughness. Therefore, it is preferred that the blend comprise constituents having molecular weights and melt viscosities similar to the polyarylate, e.g., polycarbonate. However, the relatively high melt viscosities of such blends makes them difficult to process at normal processing temperatures, i.e., normally below about 285.degree. C. It is well known that the processability of such blends can be improved if the blends are molded at higher temperatures. However, at such higher temperatures, exchange reactions, such as alcoholysis, acidolysis, and transesterification, among the polyester(s) generally take place. Such reactions may take place when one or two or more different polyesters are present in the blend. The extent of such reactions effects the ultimate properties of the blend. In transesterification, an exchange of the ester moiety between two similar or different polyester molecules takes place which generates new molecular configurations, and hence a new composition. Transesterification during the melt-mixing of such blends results in poor mechanical properties and loss of crystallinity if the polyester is a semi-crystalline material. The product becomes embrittled and loses impact strength as compared to a product made from the same polymer blend in which transesterification did not occur.
French patent 2,567,137 to Bonum and Logeat discloses incorporating esterified ortho-phosphoric acids into compositions comprising thermoplastic polyester pairs with different chemical structures, such as ethyleneglycol terephthalate and butanediol polyterphthlate-1,4 to prevent transesterification during molding thereof. It is also known that in blends containing polyesters, a residual catalyst generally remains from the polymerization which may accelerate, e.g., ester-carbonate, interchange reactions in the melt state. In Chemistry of Miscible Polycarbonate-Copolyester Blends. Smith, Barlow and Paul, Journal of Applied Polymer Science, Vol. 26, 4233-4245 (1981), it is taught that interchange reactions are greatly suppressed by deactivating the residual titanium catalyst with arsenic oxide additive. Usually, however, rather than adding compounds in an attempt to deactivate catalyts, workers in the art add any of numerous compounds to such polymer blends to improve the physical properties thereof. For example, in U.S. Pat. No. 4,066,611, it is taught that particular phosphorous compounds may be incorporated into a bisphenol A polycarbonate/poly(tetramethylene terephthalate) blend to enhance the mechanical properties of the blend. U.S. Pat. No. 4,066,611 discloses that cyclic diphosphite compounds can improve the thermal-oxidative stability and hydrolytic stability of aromatic carbonate polymer compositions. However, attempts to prevent transesterification of polymer blends containing polyester during processing at elevated temperatures above about 280.degree. C. have been less than successful.
It is an object of the present invention to provide a stabilizer capable of retarding transesterification of ester containing polymer blends during melt-mixing and molding, particularly at the higher temperatures generally necessary to effectively process such blends. We have found an effective stabilizer comprising a phosphate-epoxy adduct wherein the phosphate is selected from specifically defined mono- and di-esters of orthophosphoric acid. Use of this stabilizer in polyesters and polymer blends containing polyesters allows them to be processed at higher temperatures without any deterioration in properties since it retards transesterification even at higher working temperatures (i.e., 285.degree. C. and above). The stabilizer of this invention may comprise, in addition to this phosphate-epoxy adduct, an imide or oxazoline containing compound. The stabilizer may further or alternately comprise a hindered phenol which provides thermal stability to the blend and is particularly useful at processing temperatures above about 300.degree. C.